Solder masks provide protection to the circuitry on printed circuit boards during processing, as well as providing continued protection throughout the life of the circuit board. These masks must be resistant to solder compositions as well as etching fluids such as methylene chloride, and also withstand environmental stresses such as changes in temperature and humidity. At the same time they must remain flexible enough to withstand mechanical stresses without cracking or peeling. The Institute of Interconnecting and Packaging Electronic Circuitry (IPC) describes requirements for solder mask coatings in their SM840-B specifications. The specifications include, among other things, hydrolytic stability, electrical resistivity, thermal stability, abrasion resistance, solvent resistance, and limited flammability. The requirements for a Class III material are the most severe, since these materials are used for life support systems and military equipment.
The solder mask composition is applied to a circuit board and partially cured in the desired areas by exposure to ultraviolet radiation through a phototool or negative. After the unexposed composition is removed from the board by washing with an aqueous solution or an organic solvent, the partially cured mask is fully cured by further exposure to ultraviolet radiation or a combination of irradiation and thermal curing.
Aqueous developable liquid solder mask compositions have been described, for example, in U.S. Pat. Nos. 4,666,821, 4,761,363 and 4,789,620. Compositions such as these do not contain solvents and therefore remain in the liquid state until cured. They require special process equipment and special processing steps such as off-contact printing. Other liquid, aqueous developable solder mask compositions are described in U.S. Pat. Nos. 4,438,189; 4,621,043 and 4,695,527. All of these compositions, however, require thermal curing in addition to irradiation with ultraviolet light to completely cure the mask. Compositions requiring a thermal cure contain thermally activated cross-linking agents that can cause thermal instability prior to and during drying and imaging. U.S. Pat. No. 4,717,643 describes an aqueous developable, dry film solder mask that does not require a thermal cure. However, the synthesis of the binder requires the grafting of isocyanatoethyl methacrylate groups onto the acrylic backbone polymer, which is a complicated and time consuming procedure. Vacuum lamination of the film to the substrate is also required.